configure: prepare for auto-generated option parsing

configure: prepare for auto-generated option parsing

Prepare the configure script and Makefile for automatically generated
help and parsing.

Because we need to run the script to generate the full help, we
cannot rely on the user supplying the path to a Python interpreter
with --python; therefore, the introspection output is parsed into
shell functions and stored in scripts/.  The converter is written
in Python as standard for QEMU, and this commit contains a stub.

Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211007130829.632254-18-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>